Field Of Utilization In Industry
The present invention relates to a liquid crystal display of an active matrix type employing thin-film transistors (TFTs), and a method of fabricating such a liquid crystal display.
In an active matrix type liquid crystal display, pixel electrodes arranged in a matrix area are connected respectively to switching elements. Since portions of the liquid crystal material corresponding to the pixels of the active matrix liquid crystal display are driven theoretically in a continuous manner, the contrast of a picture displayed on the active matrix liquid crystal display is higher than that displayed on the simple matrix liquid crystal display. Such an advantage of the active matrix liquid crystal display is essential to displaying color pictures.
In a TFT incorporated into the conventional active matrix liquid crystal display, gate lines (gate electrode) are formed on an insulating substrate, a gate insulating film is formed over the gate lines, a semiconductor layer is formed over the gate insulating film, a drain electrode and a source electrode are formed on the semiconductor layer, a transparent pixel electrode is connected to the source electrode, and a data signal, i.e., a voltage signal, is applied to the drain electrode. The construction of a TFT having a gate electrode formed directly on the substrate is generally referred to as an inverted staggered construction. Such an active matrix liquid crystal display is disclosed in Japanese Patent Laid-open (Kokai) No. 62-32651.
Although the liquid crystal display employing TFTs is capable of being driven in an active drive mode and hence is capable of displaying pictures in a high contrast, the formation of the TFTs on the substrate requires complicated processes, including six to nine photolithographic etching processes. These many complicated processes increase the manufacturing cost of the substrate provided with TFTs and reduces the yield of the TFT production line, because an increase in the number of processes increases the possibility of contamination by dust and foreign matter.
A method proposed to simplify the TFT fabricating processes involves formation of the gate insulating film and the semiconductor layer simultaneously in the same pattern However, this known method has problems in that defects in the lines due to a short circuit between the drain electrodes or the source electrodes, the gate lines increase if the drain electrodes (data lines) or the source electrodes are not formed in a correct pattern, the breakage of the data lines at steps increases because the semiconductor layer and the gate insulating films are patterned in lands and form steps, the formation of the pixel electrodes and the gate lines and the data lines on the same layer increases defects attributable to defective points, and leakage currents increase to deteriorate the quality of displayed pictures when the semiconductor layer is formed of amorphous silicon having a high light sensitivity and the semiconductor layer is illuminated for backlighting.